Amphiregulin (AR) is a secreted heparin binding growth factor that is structurally and functionally related to epidermal growth factor (EGF) and transforming growth factor alpha (TGFalpha). GEO and WIDR cells are human colon cancer cell lines. Northern blot analysis of poly (A) mRNA showed that GEO cells but not WIDR cells express a 1.4kb specific AR transcript. However, endogenous AR protein could be immunocytochemically detected in both cell lines, with GEO exhibiting a higher level of staining than WIDR cells. Two different phosphorothioate 20-mer antisense oligonucleotides complementary to the 5' sequence of AR mRNA were able to inhibit the growth of GEO and WIDR cells. The two antisense oligonucleotides at a concentration of 10 uM inhibited the soft agar growth of GEO and WIDR cells by 90% and 50%, respectively, as compared to the untreated control cells. A missense oligonucleotide had no effect on the growth of the two cell lines. An inhibition of 50% of monolayer growth of GEO cells treated with the same concentration of antisense was observed. These data suggest that AR may play a role as an autocrine growth factor for human colon carcinoma cells.